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Desulphurization facility for Unit 4

The desulphurization facility at generating unit four is connected to the unit four boiler which uses lignite coal as fuel.

The flue gasses that exit the boiler contain SO2, CO2, NOx, dust, ash and other materials. Two draught fans draw the flue gasses from the boiler via two channels to the electrostatic filters where more than 99.8 percent of the ash is deposited. The flue gasses cleared of the particles can be directed in two directions by the draught fans. The normal route is through the desulphurization system but in emergencies, they can be conveyed directly into the chimneystack via the bypass hatches.

During normal operation, the flue gasses are passed through a heater named GAVO, while the bypass hatches are closed. The GAVO is a regenerative rotating flue gas heater in which the unfiltered flue gasses with a temperature of 160 °C transfer their heat to the heating banks, and these then return the heat to the filtered flue gasses, before entry into the chimneystack to keep their temperature above dew point (90-95 °C).

The generating unit four desulphurization facility also receives, when possible, flue gasses from units 1-3 which do not have a dedicated desulphurization facility.
Unfiltered flue gasses from the flue gas heater enter the scrubber at a temperature of 120 °C, where they are sprayed with a suspension of the absorption material (ground limestone; CaCO3 + water). The part of the scrubber above the level of the suspension has absorption material spray nozzles fitted in six levels. Each system of nozzles has an adjoining pump, which pumps the suspension from the scrubber’s collector, and the nozzles guarantee a fine spray. The cooling of the flue gasses and an essential phase in the filtration process, the diffusion of the sulphur dioxide, fluorides and chlorides through the bordering gas strata onto the surface of the wet particle of the absorbent occurs in the area of the spray nozzles.

The flue gasses moisten to a point of saturation and cool down to 60 °C in the scrubber. Because they contain finely sprayed droplets, they must travel through the droplet separator at the top of the scrubber where the droplets are separated along with other crystallization cores. The gasses partially vaporize the suspension when passing through the scrubber, and the vapours exit the system. The steam lost in this process is replaced by additional water, with which we wash the separator and the processing water reservoirs. An induced draught fan that is built into the flue gas channel raises the pressure of the filtered flue gasses that exit the droplet separator.  The fan propels the filtered gasses from the scrubber over the flue gas heater and the hatches to the chimneystack. The calcium sulphite, which is produced in the spray area of the scrubber, falls into the collector. There it has enough time to oxidise into calcium sulphate. There are three blowers for the oxidation process, which supply the necessary quantity of air into the suspension with the calcium sulphite, which oxidizes into calcium sulphate because of the mixing and the intense influx of air. For an efficient process of elimination of SO2 from the flue gasses, it is very important to maintain a pH between 5.7 and 6.0, along with the right density and temperature.

The ground limestone, which is used as the absorption material in the desulphurization process, is obtained from the local silos, under which two reservoirs for the preparation of the limestone suspension are located. The water needed for the suspension flows from the circulation water reservoir.

The limestone suspension, which turns into gypsum in the scrubber, is pumped into the drainage station where it is condensed on the hydro-cyclone. The condensed gypsum is conveyed to a produce reservoir and pumped through a circular pipe system into the produce mixer where it is mixed with ash. The ash is pneumatically transported from below the electrostatic filter into the 2,000 cubic meter ash silo. The ash and the gypsum are dosed in the mixer and after the mixing, we have a final produce with stable physical and chemical properties. This produce is then conveyed to a temporary dump where it is loaded onto transport vehicles and driven to final repository locations.